Kid has a costly hobby that's making him money. Do I charge him? What's fair?

A little background. Our 11 year old ADHD'er has had hobbies come and go (and come and go and come and go, as we ADHD'ers are wont to do). None have really stuck long, except video games. Long and short of it, I bought some Perler (plastic fuse beads) for his younger sibs, but he took a shine to them.He's been making exceptionally cool pieces. Moving from simple designs to large pieces, intricate, multi-pieced forms, 3d pieces. He's sold quite a few, making a bit of cash. His hobby has cost me over $200, and made him getting close to half of that.I want to encourage and support his healthy and creative hobby. It's time away from the screen, often we spend it together. He has to think critically to solve problems. He's being entrepreneurial. But I'm not exactly flush and it's getting a little expensive. Plus I want him to learn the a business does have a bottom line.Should I keep it free for him a bit longer (it's only been around 2 months or so, just over summer)? Or charge a percentage? I have a tendency to be easier on him, he's got some lifelong life-threatening illnesses, so I can be a bit soft.Thanks for the input! via /r/Parenting https://ift.tt/2HIIgWO